The present application relates to lift cranes, and particularly to mobile lift cranes having a counterweight that can be moved to different positions in an effort to balance a load on the crane.
Lift cranes typically include counterweights to help balance the crane when the crane lifts a load. Sometimes the counterweight on the rear of the crane is so large that the carbody is also equipped with counterweight to prevent backward tipping when no load is being lifted. Further, an extra counterweight attachment, such as a counterweight trailer, is sometimes added to the crane to further enhance the lift capacities of the mobile lift crane. Since the load is often moved in and out with respect to the center of rotation of the crane, and thus generates different moments throughout a crane pick, move and set operation, it is advantageous if the counterweight, including any extra counterweight attachments, can also be moved forward and backward with respect to the center of rotation of the crane. In this way a smaller amount of counterweight can be utilized than would be necessary if the counterweight had to be kept at a fixed distance.
Since the crane needs to be mobile, any extra counterweight attachments also need to be mobile. However, when there is no load on the hook, it is customary to support these extra counterweights on the ground apart from the main crane; otherwise they would generate such a moment that the crane would tip backward. Thus, if the crane needs to move without a load on the hook, the extra counterweight attachment also has to be able to travel over the ground. This means that the ground has to be prepared and cleared, and often timbers put in place, for swing or travel of the extra counterweight unit.
A typical example of the forgoing is a TEREX® DEMAG® CC8800 crane with a Superlift attachment. This crane includes 100 metric tonne of carbody counterweight, 280 metric tonne of crane counterweight, and 640 metric tonne on an extra counterweight attachment, for a total of 1020 metric tonne of counterweight. The extra counterweight can be moved in and out by a telescoping member. This crane has a maximum rated load moment of 23,500 metric tonne-meters. Thus the ratio of maximum rated load moment to total weight of the counterweight is only 23.04.
While all of this counterweight makes it possible to lift heavy loads, the counterweight has to be transported whenever the crane is dismantled for moving to a new job site. With U.S. highway constraints, it takes 15 trucks to transport 300 metric tonne of counterweight. Thus there is a need for further improvements in mobile lift cranes, where the same large loads can be lifted using less total crane counterweight.